I'm guessing it's just up to us, although within the realms of believability if possible. Obviously some of the starters evolve a lot earlier than the others, and some [Such as Eevee, luck Ffion eh?] could be evolved via evolution stones.
Although that's not going to happen, it may be a while before I have Ffion's Eevee evolve, although I have semi-plans for that already.
I probably shouldn't plan that far ahead, heh.